The present invention relates to a method for producing fatty acid alkyl esters, involving transesterifying a feedstock containing lipid-linked fatty acids with an alcohol and an alkaline catalyst to form fatty acid alkyl esters. Generally, the feedstock has not been previously treated to release the lipid components of the feedstock, or the feedstock has been previously treated to release lipid components yet the feedstock contains residual lipids (e.g., <about 30% of the original content of lipids).
Over the past three decades interest in the reduction of air pollution, and in the development of domestic energy sources, has triggered research in many countries on the development of non-petroleum fuels for internal combustion engines. For compression ignition (diesel) engines, it has been shown that the simple alcohol esters of fatty acids (biodiesel) are acceptable alternative diesel fuels. Biodiesel has a higher oxygen content than petroleum diesel, and therefore reduces emissions of particulate matter, hydrocarbons, and carbon monoxide, while also reducing sulfur emissions due to a low sulfur content (Sheehan, J., et al., Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in an Urban Bus, National Renewable Energy Laboratory, Report NREL/SR-580-24089, Golden, Colo. (1998); Graboski, M. S., and R. L. McCormick, Prog. Energy Combust. Sci., 24:125-164 (1998)). Since it is made from agricultural materials, which are produced via photosynthetic carbon fixation (e.g., by plants and by animals that consume plants), the combustion of biodiesel does not contribute to net atmospheric carbon levels.
Initial efforts at the production, testing, and use of biodiesel employed refined edible vegetable oils (expelled or recovered by solvent extraction of oilseeds) and animal fats (e.g., beef tallow) as feedstocks for fuel synthesis (Krawczyk, T., INFORM, 7: 800-815 (1996); Peterson, C. L., et al., Applied Engineering in Agriculture, 13: 71-79 (1997); Holmberg, W. C., and J. E. Peeples, Biodiesel: A Technology, Performance, and Regulatory Overview, National SoyDiesel Development Board, Jefferson City, Mo. (1994); Freedman, B., et al., J. Am. Oil Chem. Soc., 61(10): 1638-1643 (1984)). More recently, methods have been developed to produce fatty acid methyl esters (FAME) from cheaper, less highly refined lipid feedstocks such as spent restaurant grease and soybean soapstock (Mittelbach, M., and P. Tritthart, J. Am Oil Chem. Soc., 65(7):1185-1187 (1988); Graboski, M. S., et al., The Effect of Biodiesel Composition on Engine Emissions from a DDC Series 60 Diesel Engine, Final Report to USDOE/National Renewable Energy Laboratory, Contract No. ACG-8-17106-02 (2000); Haas, M. J., et al., Enzymatic Approaches to the Production of Biodiesel Fuels, in Kuo, T. M. and Gardner, H. W. (Eds.), Lipid Biotechnology, Marcel Dekker, Inc., New York. (2002); Canakci, M., and J. Van Gerpen, Biodiesel Production from Oils and Fats with High Free Fatty Acids, Abstracts of the 92nd American Oil Chemists' Society Annual Meeting & Expo, p. S74 (2001); U.S. Pat. Nos. 2,383,601; 2,494,366; 4,695,411; 4,698,186; 4,164,506; Haas, M. J., et al., J. Am. Oil Chem. Soc., 77:373-379 (2000); Haas, M. J., et al., Energy & Fuels, 15(5):1207-1212 (2001)).
One aspect of the present invention is the production of fatty acid alkyl esters using as substrate unextracted lipids still residing in the agricultural materials in which they were produced. Our method achieved the desired transesterification of the lipid-linked fatty acids by direct treatment of the lipid source itself with alcohol and an alkaline catalyst. Because no prior isolation or purification of the lipid in the lipid source was involved, this method for ester synthesis should have a greatly reduced cost compared to existing methods since it eliminates the need for costly expelling/extraction and refining steps currently employed to produce the fats and oils that are the feedstock for fatty acid ester synthesis. Our method can also be used with a feedstock that has been previously treated to release lipid components yet contains residual lipids (e.g., <about 30% of its original content of lipids).